As the size of devices such as personal computers, video cameras, and mobile phones has been becoming smaller, lithium secondary batteries have been put into practical use and have been popularized as power sources used in these devices in the fields of information technology-related devices and communication devices since the batteries have a high energy density. In the meantime, in the field of automobiles also, the development of electric vehicles has been hastened from the viewpoint of environmental problems and resource problems. As power sources of the electric vehicles also, lithium secondary batteries have been investigated.
However, a high-capacity lithium secondary battery with metallic lithium used as an anode or the like has not been put into practical use although attention has been paid thereto as a secondary battery exhibiting a high energy density. In other words, the following problem has remained: the surface of its metallic lithium foil piece is not smooth so that the surface has spots where an electric field is concentrated; this causes the lithium metal to grow into a dendrite form by charging and discharging the battery repeatedly; and thus an internal short circuit is caused between the anode and the cathode so that the cycle characteristics decline.
In lithium secondary batteries commercially available at present, an organic liquid electrolyte, in which an organic solvent is used as a solution medium, is used. In such a lithium secondary battery, its cathode active material and its liquid electrolyte contact each other to react with each other. Thus, when the battery is repeatedly charged and discharged, the cathode active material and the liquid electrolyte are deteriorated so that the charging and discharging electricity quantities are decreased. As a result, there arises a problem that the cycle characteristics decline.
In order to improve the durability and the cycle characteristics of such lithium secondary batteries, for example, Patent Document 1 discloses a secondary battery wherein at least a surface of a cathode that is opposite to an anode is covered with one or more thin films each selected from an insulator which ions related to the battery reaction can permeate, a semiconductor, and such an insulator and a semiconductor. This is a battery wherein the cathode surface is covered with the thin film(s) of the electron-nonconductive insulator, which ions related to the battery reaction can permeate, or with the semiconductor, thereby preventing short circuit between the anode and the cathode inside the battery at the time of the generation of dendrites in the anode, so as to improve the cycle characteristics. However, according to Patent Document 1, the surface is covered with the thin film(s) of the electron-nonconductive insulator; therefore, the shift of electrons becomes difficult and others are caused, so as to cause a problem that the output characteristics of the lithium secondary battery decline.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. Hei 6-168739
Patent Document 2: JP-A No. 2006-216277
Patent Document 3: JP-A No. Hei 10-321216